Referring to FIG. 1 herein, a known data storage format for writing data to a tape data storage medium has a logical data subgroup 100 known as a “G4 subgroup” comprising an array of data bytes arranged in rows and columns, there being 96 columns and 124 rows in a data group, each column being referred to as a “data fragment”. Each column comprises 124 bytes of data numbered 0 to 123. Each row comprises 96 bytes, numbered 0 to 95.
The data subgroup has applied to it redundancy error correction coding (ECC). The applied redundancy error correction coding, which may, for example, be a Reed-Solomon coding, comprises 192 C1 code words, each column comprising a pair of interleaved C1 code words and 336 C2 code words extending over 112 of the rows. Each coded row comprises three interleaved C2 code words, each code word having 32 bytes. Shown in FIG. 1 is an example of first row 101 (i.e., row having number 0), comprising an interleaving of three C2 code words in row 0, where bytes of three code words A, B, C respectively of the first row are interleaved alternately such that individual bytes are ordered A, B, C, A, B, C, along the row. C2 code words of an additional 111 rows are arranged similarly.
Since each column comprises two C1 code words, the G4 data subgroup comprises 96×2=192 C1 code words. C1 code words run vertically down the columns, whilst C2 code words run horizontally across rows of the array. Each C1 code word extends along the entire height of its column, and each C2 code word extends across the entire width of its row. The C2 code words occupy rows 0-111, so that there are 112×3=336 C2 code words in the G4 data subgroup.
Referring to FIG. 2 herein, the entire G4 data subgroup is stored on a magnetic tape data storage medium in a single diagonal track 200 extending transversely and lengthwise of the tape as a stripe. In the prior art format, the physical width of the track a is 6.8 μm, the track density is 147 tracks/mm (3,735 tracks/inch), and the density of bits written along the tracks is 4,800 bits/mm (122,000 bits/inch). This results in a data storage capacity of 20 Gbytes of uncompressed data (40 Gbytes at 2:1 compression ratio) in a single data storage cartridge, containing a tape data storage medium of length 150 m. The mean time between failure (MTBF) reliability parameter of the prior art system, which is related to tape thickness, is 250,000 hours at a 30% duty cycle.
Data are written sequentially in parallel diagonal tracks as the tape moves past a write head. The plurality of tracks abut each other, so as to store a maximum amount of data on the tape. At a start of each track is provided a first synchronization tone 201 which allows a phase locked loop (PLL) to synchronize with the data. Similarly, at the end of each track is provided a second set of tones 202. A first entire G4 data subgroup is stored between the first and second tone regions in a single diagonal track. A second entire G4 data subgroup is stored between the tone regions of a second single diagonal track having an edge abutting an edge of the first track.
Due to increased data storage demands, ongoing objectives in the improvement of data storage devices include:
Increasing the amount of data which can be stored on a data storage medium;
Increasing the data rate for writing data to a data storage medium;
Improving reliability and byte error rates.
The above 3 parameters are interrelated, and attempts at increasing the amount of data storage capacity can affect the reliability of a tape data storage system, particularly a tape data storage system.